A high-power discrete transistor consists of an active transistor or a parallel combination of transistors, along with impedance matching, and may be interfaced to the RF PA (power amplifier) circuit environment through a package with leads. Internal impedance matching inside the package comprises capacitors and inductors, which are used to transform from low impedance at the gate or drain of the transistor (for a common-source configuration) level to higher impedance at the package leads.
In a traditional high-power discrete transistor, the output capacitance between the drain-terminal and the source-terminal limits the bandwidth of the high-frequency power-amplifier, and can degrade performance characteristics such as output power, power gain and efficiency. A well-known technique to mitigate the adverse effects of output capacitance is to use a shunt inductor in parallel with the output capacitance.
However, connecting a shunt inductor directly between the source and drain terminals of the transistor creates a short circuit to the DC current flowing between the drain and source terminals. This DC short can be eliminated by placing a capacitor or a DC block between the shunt inductor and the electrical ground. The DC block should ideally act as a short circuit at high frequencies. This type of circuit is traditionally referred to as an integrated-shunt inductor or “Inshin” and is shown in FIG. 1.
More specifically, FIG. 1 shows a field-effect transistor (FET) 105 with terminals G (gate), D (drain) and S (source). The output capacitance of the FET 105, i.e. the capacitance between the D and S terminals, is CO. The Inshin network 110 is arranged in parallel with the output capacitance CO and consists of an inductor L1 connected in series with a capacitor C1.
However, using an Inshin matching network at the output of a power transistor only provides impedance matching at a single frequency band with a limited bandwidth.
There may thus be a need for an improved discrete power transistor, in particular for a discrete power transistor with an improved output matching topology.